Non-aqueous electrolyte secondary batteries, particularly lithium ion secondary batteries, now have a high operating voltage and high energy density. They are commercialized as power sources for driving portable electronic equipment such as cell phones, notebook computers and video camcorders.
Lithium ion secondary batteries employ, as the positive electrode active material, a transition metal-containing composite oxide having a voltage as high as 4 V level. Specific examples of the positive electrode active material include: lithium-cobalt composite oxides having a hexagonal crystal structure (e.g., LiCoO2 and one obtained by partially replacing Co in LiCoO2 with Mg or Al); lithium-nickel composite oxides (e.g., LiNiO2 and one obtained by partially replacing Ni in LiNiO2 with Co, Al or Mn); lithium-manganese composite oxides having a spinel structure (e.g., LiMn2O4 and one obtained by partially replacing Mn in LiMn2O4 with Cr, Al or Ni); and lithium-titanium composite oxides (e.g., Li4Ti5O12). A mixture composed of a plurality of composite oxides is also used. Among the above, the most widely used is LiCoO2 because it can offer a high operating voltage and a high energy density.
Lithium ion secondary batteries employ, as the negative electrode active material, a material capable of absorbing and desorbing lithium ions. The most widely used is graphite because it can provide a flat discharge potential and a high capacity density.
These active materials are added with a binder such as polyvinylidene fluoride or polytetrafluoroethylene, and optionally, added with a conductive material such as acetylene black or graphite. A paste is prepared by mixing the above materials with a liquid component. The paste is applied to a metal foil made of aluminum or copper, followed by drying and rolling to produce an electrode plate. The electrode plate is then cut into a predetermined size to produce a sheet-like electrode.
In addition to the compact non-aqueous electrolyte secondary batteries for consumer use, the development of large non-aqueous electrolyte secondary batteries having a large capacity has also been accelerated in recent years. Particularly, the development of lithium ion secondary batteries for power storage and electric vehicle applications is being vigorously conducted. Currently, hybrid electric vehicles (HEVs) are considered promising in view of an environmental friendliness. Vehicles equipped with on-board nickel-metal hydride storage batteries have already been in mass production and available in the market. Trials have been vigorously made to combine a lithium ion secondary battery having a higher energy density than a nickel-metal hydride storage battery with a conventional engine or a fuel cell.
Unlike the compact lithium ion secondary batteries for consumer use, those for HEV application need to exhibit stable storage characteristic in any outside environment (in a high temperature environment of a hot summer day, in particular). Lithium ion secondary batteries for HEV application also need to instantly provide power assistance (output) to the main power source (e.g., an engine or fuel cell), or to regenerate power (input). For this reason, there is a growing demand for an electrode structure designed for high input/output application with a small internal resistance.
Under the circumstances, in order to improve storage characteristic, a positive electrode active material comprising primary particles of not greater than 2 μm and having a pore radius of not greater than 30 Å is proposed (Japanese Laid-Open Patent Publication No. Hei 9-231973). This proposal is intended to prevent the decomposition of organic solvent which selectively takes place in a void having a pore radius of not greater than 30 Å and to prevent the resulting decomposition product from covering the active material.
In order to improve high output characteristic, a positive electrode active material comprising secondary particles having an average particle size of 5 to 15 μm, each comprising an aggregate of primary particles having an average particle size of 0.3 to 1 μm is proposed. The positive electrode active material is represented by Li(Ni—Co-M)O2, where M is at least one selected from the group consisting of Al, Ti and Sn. This proposal is intended to reduce the internal resistance of the non-aqueous electrolyte secondary battery (Japanese Laid-Open Patent Publication No. 2004-87492).
The proposal made by Japanese Laid-Open Patent Publication No. Hei 9-231973 is effective in improving storage characteristic only when the battery is deeply charged at a relatively low rate until the battery voltage reaches 4.2 V. However, for HEV application, about 100 batteries are typically connected in series, and each battery is not deeply charged to a voltage of 4.2 V. Such batteries for HEV application are typically charged at a rate 10 times or more higher than the compact batteries for consumer use. Accordingly, the state of charge (SOC) of each battery is further reduced due to polarization. When such batteries that are commonly charged to a low SOC are stored at a high temperature, the proposal of Japanese Laid-Open Patent Publication No. Hei 9-231973 cannot provide sufficient improvement of the storage characteristic.